Procalcitonin can help differentiate bacterial from viral CAP, but no specific cutoff
found

For distinguishing any type of bacterial community-acquired pneumonia (CAP) from viral
CAP, a procalcitonin threshold of 0.1 ng/mL produced a sensitivity of 80.9% and a
specificity of 51.6%.

Procalcitonin values demonstrated accuracy in differentiating viral from bacterial
community-acquired pneumonia (CAP) in a recent study, but not at clear-cut thresholds.

Researchers used data from 1,735 adults hospitalized with CAP (median age, 57 years)
who were enrolled in the CDC Etiology of Pneumonia in the Community study from January
2010 through June 2012. Participants had procalcitonin measurements and underwent
systematic bacterial and viral pathogen testing.

Based on microbiology test results, researchers categorized patients into five pathogen
groups: typical bacteria (169 patients [10%]), atypical bacteria (67 [4%]), virus
(409 [24%]), mycobacteria/fungus (15 [1%]), and no pathogen detected (1,075 [62%]).
They conducted three analyses to assess the accuracy of procalcitonin in identifying
bacterial CAP, comparing 1) patients with any bacterial pathogen to those with a viral
pathogen, 2) patients with typical bacteria to those with atypical bacteria or viruses,
and 3) patients with any bacterial pathogen to those who did not have bacteria detected.

Results were published online on April 12 by Clinical Infectious Diseases.

Researchers used the following procalcitonin values, which have been described in
the literature as thresholds for identifying bacterial CAP: <0.1 ng/mL, 0.1 to
0.24 ng/mL, 0.25 to 0.49 ng/mL, and ≥0.5 ng/mL.

Median procalcitonin was lower in the viral group compared to the atypical bacterial
group (0.09 ng/mL vs. 0.20 ng/mL, P=0.05) and typical bacterial group (2.5 ng/mL, P<0.01). Typical bacteria were more prevalent in patients with higher procalcitonin
concentrations (3% among those with <0.1 ng/mL vs. 21% among those with ≥0.5
ng/mL). Among patients with typical bacteria, 23.1% had procalcitonin <0.25 ng/mL,
and 12.4% had procalcitonin <0.1 ng/mL.

For distinguishing any bacterial CAP from viral CAP, a procalcitonin threshold of
≥0.1 ng/mL produced a sensitivity of 80.9% (95% CI, 75.3% to 85.7%) and a specificity
of 51.6% (95% CI, 46.6% to 56.5%). For distinguishing typical bacterial CAP from viral
and atypical CAP, the same threshold produced a sensitivity of 87.6% (95% CI, 79.6%
to 90.7%) and a specificity of 49.3% (95% CI, 44.8% to 54.0%). For bacterial CAP versus
nonbacterial CAP, sensitivity was 80.9% (95% CI, 75.3% to 85.7%) and specificity was
46.2% (95% CI, 43.7% to 48.8%).

The study authors noted limitations, such as how almost a quarter of patients from
the CDC study were excluded because procalcitonin measurements were not available
and that, for those with values, the biomarker was only measured at the time of admission.
They wrote that procalcitonin concentrations “could be a useful adjunct in
the etiologic assessment of patients hospitalized with CAP,” but added that
“clinicians cannot rely on procalcitonin alone to guide antibiotic decisions.”

Procalcitonin appears to correlate with viral or bacterial etiologies of acute respiratory
tract infections, “but not terribly well,” according to an accompanying editorial. However, the biomarker can likely be used to help guide clinical management, the
editorialists stated. “The challenge going forward is learning how to reconcile
host and pathogen-based diagnostics to gain a comprehensive understanding of the patient's
disease,” they wrote.

ACP Hospitalist provides news and information for hospitalists, covering the major issues in the field. All published material, which is covered by copyright, represents the views of the contributor and does not reflect the opinion of the American College of Physicians or any other institution unless clearly stated.